1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular to a technique for controlling power supply to an integrated circuit constituting an image forming apparatus.
2. Description of the Related Art
In recent years, controller systems that use an integrated circuit called a system-on-a-chip (SOC) to control overall operation of an image forming apparatus have become widespread. As the SOC, one which incorporates an analog front-end (AFE) that converts an analog image signal into a digital signal is becoming common. The AFE has a filter circuit that removes noise from a read analog signal, an amplification circuit that amplifies an analog signal, and an A/D converter that converts an analog signal into a digital signal.
Power to a reading unit, which is comprised of an AFE and a reading sensor such as a CIS or a CCD, is supplied from an accurate series path regulator (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. H10-304134). A DC-DC converter is a power supply that has higher power conversion efficiency than that of the series path type regulator, but use of the DC-DC converter may cause ripple noise in output from the DC-DC converter due to the effect of switching. For example, when power is supplied from the DC-DC converter to the reading unit, noise may affect an image in, for example, a mode where a low-intensity halftone image is read, causing image quality to deteriorate.
An AFE and a reading sensor are usually connected together using a long cable since they are placed away from each other. Thus, there is concern that the ripple noise could make radiation noise worse. Further, in an image forming apparatus that carries out facsimile communications through an analog public line, undesired noise may propagate to the analog public line. On the other hand, from the standpoint of saving energy for an image forming apparatus, a DC-DC converter is more advantageous than a series path regulator.
An SOC with a built-in AFE is subjected to the constraint that power to the AFE cannot be turned off while an image forming apparatus is in energy-saving mode (sleep mode). On the other hand, the SOC has an IO circuit or the like as well as the AFE, and hence, for example, an interrupt signal for returning from a sleep state is input to the SOC. To make this interrupt signal valid, it is necessary to supply power to the IO circuit in the SOC when the image forming apparatus is in sleep mode. When power to the AFE is turned off in sleep mode, a potential difference occurs between a circuit to which power is being supplied and the AFE. As a result, current flows from the circuit to which power is being supplied to the AFE to which no power is being supplied, and this causes a latch-up phenomenon or the like and may damage a device.
As described above, when an image forming apparatus is in sleep mode, power must be supplied to an AFE. Also, from the standpoint of ensuring image quality, power to an AFE needs to be supplied from a series path regulator. The series path regulator lowers voltage using a transistor or a field-effect transistor (FET) and obtains desired output voltage with respect to input voltage.
For this reason, the series path regulator does not perform switching as distinct from a DC-DC converter, and hence it is less affected by noise. On the other hand, the series path regulator basically matches output voltage to desired output voltage by consuming power with a transistor or an FET and is thus inefficient in power conversion and not suitable for energy saving.